The plain TeX code below might not be a minimal example, but it's
self-documenting!
Stan
%%%%%%%%%%%%
This sentence's length and the use of a mark create an unexpected blank
line below for some reason..... \mark{mark}
Removing one period after ``reason'', or the space before the mark, makes
the blank line disappear. (I'm not saying that putting a space there makes
sense.) Does anyone know why this happens? My only guess is that the mark
prevents the space before it from being removed when TeX breaks the text
into lines. I'm not sure if this is a bug, but it seems odd, at least.
\bye